A spider party is when all your mates are going out for a night out and one mate doesn't go because hes stuck in with his bird, so therefore hes says hes got some other party arranged also known as the spider party.

Mate1: Hey man we have organised this fuckin great night out, out going?
Mate2: erm... dunno
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Mate1: Yeh i just asked him and he said dunno which means he gonna give us a spider party excuse
Mate3: Thats allways the case!
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Mate1: So you gonna come then?
Mate2: Sorry cannot i got this other party.
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Mate2: HIM AND HIS FUCKIN SPIDER PARTYS!